The authors of the publication WO 02/101120 disclosed the method of how to obtain bulk mono-crystalline gallium-containing nitride through crystallization on the seed from the super-critical solution containing a nitride-containing solvent, preferably ammonia. By this method it is possible to obtain bulk mono-crystalline gallium-containing nitrides of a higher quality of parameters compared to the substrates used in industry, which are obtained by deposition methods from the gaseous phase, such as HVPE and MOCVD or MBE, i.e., of lower dislocation density than those substrates. Mono-crystals obtained by the method as known from the disclosure of WO 02/101120 show high increments in volume. Due to equilibrium character of the crystallization process, a very high crystalline quality of single crystals is attained from the solution containing a supercritical gallium-containing solvent, compared to the materials used industrially in various centers worldwide. The primary advantage of the technology known from WO 02/101120 is that it can assure a convenient range of pressures and temperatures, in which the re-crystallization process of gallium-containing nitride from the supercritical solution based on the nitride-containing solvent takes place.
In course of further research and developmental work over the method disclosed in WO 02/101120 a number of factors, which had a limiting impact on practical application of that method, were recognized and gradually the encountered barriers were surmounted, both in terms of technology and apparatuses. Some of such barriers are: a limited availability of the feedstock of desired purity, proper quality of crystalline seeds, selection of proper mineralizers and control of the growth rate of single crystals.
Other methods of synthesis of gallium-containing nitrides, such as HNP, are also known. By those methods gallium-containing single crystals of very high crystalline quality and low dislocation density are obtained. Unfortunately, because of the unsatisfactory size and irregular shape of crystals which are obtained thereby, they have not been used so far as the material for the substrate for epitaxy in industrial production of LEDs, LDs and other semiconductor structures. Also, parameters of the process, and in particular the necessity of using very high pressures, significantly limit the feasibility of obtaining crystals of a desired size by this method on an industrial scale.
The studies in that filed show promising results obtained with the use of flux methods of growth of gallium-containing nitrides from a gallium melt in the atmosphere of nitrogen. Those processes are attractive industrially because relatively low temperatures and pressures are used.
The basic starting materials for the process disclosed in WO 02/101120, i.e., the feedstock subject to re-crystallization and the seeds, were obtained by HVPE method, according to which mono-crystalline gallium nitride layers from the gaseous phase are placed on hetero-seeds, in particular on sapphire. As the result of differences between lattice constant of the hetero-seed and the obtained bulk mono-crystalline gallium-containing nitride, as well as the result of differences in thermal expansion of both materials, bulk gallium-containing nitride single crystals, preferably gallium nitride single crystals, obtained by HVPE method have a disordered crystalline structure, which is reflected, e.g., in small radius of curvature of the obtained bulk mono-crystalline gallium-containing nitrides. The use of such single crystals as crystallization seeds in the process of re-crystallization of mono-crystalline gallium-containing nitride from the supercritical ammonia-containing solution leads to propagation of crystalline defects and surface dislocations in mono-crystalline gallium nitride layers obtained on such seeds. Additionally, different conditions of growth on the gallium-terminated and nitrogen-terminated sides of seeds were observed on seeds in the form of wafers oriented perpendicularly to the c axis of the gallium nitride crystalline lattice.
The authors of the publication WO 03/035945 disclosed that it was possible to effectively improve the quality of crystallization seeds by way of covering them with the ELOG structures having surfaces susceptible to lateral growth, i.e., toward the a axis of the crystalline lattice of gallium-containing nitride, i.e., in accordance with the methods of quality improvement of substrates for epitaxy obtained by the methods of growth from the gaseous phase. Considering, however, random arrangement of crystalline defects and surface dislocations, it is impossible, on the seeds covered with ELOG structures, to eliminate in sufficient degree the propagation of crystalline defects of the primary substrate, obtained by HVPE method, to mono crystalline gallium nitride layers deposited from the supercritical solution based on nitride-containing solvent. Surfaces susceptible to lateral growth arranged in small distances from each other are separated by strips grown directly on the primary substrate. It must be noted at this point that multiplied and alternate deposition of the ELOG structures on crystalline seeds cannot be taken into account mainly because of high costs.